The present invention relates to liquid, urethane-free, allophanate-modified, diphenylmethane diisocyanates (MDI) and to processes for their production and use.
Liquid diphenylmethane diisocyanates are generally known in the art. U.S. Pat. No. 3,644,457, for example, discloses room temperature, stable, liquid isocyanates derived from one mole of diphenylmethane diisocyanate and 0.1 to 0.3 moles of poly-1,2-propylene ether glycol.
U.S. Pat. No. 4,055,548 discloses liquid isocyanate prepolymer compositions obtained by reacting polymethylene polyphenylisocyanate containing from about 65 to 85 percent by weight of methylene bis-phenylisocyanate with a polyoxyethylene glycol having molecular weight of from 200 to 600 in an equivalent ratio of 0.0185 to 0.15:1.
U.S. Pat. Nos. 4,115,429 and 4,118,411 disclose low temperature (i.e., down to -5.degree. C.), storage stable liquid diphenylmethane diisocyanates which are produced by reacting diphenylmethane diisocyanates having a specified 2,4'-isomer content with propylene glycol or poly-1,2-propylene ether glycol.
U.S. Pat. No. 4,261,852 discloses liquid polyisocyanate compositions made up of 50 to 90% by weight of a reaction product of diphenylmethane diisocyanate and a polyoxypropylene diol or triol (hydroxyl equivalent weight of from 750 to 3000) having an NCO content of from 8 to 26% by weight, from about 10 to 50% by weight of a diphenylmethane diisocyanate containing from 30 to 65% by weight of diphenylmethane diisocyanate, the remainder being polymethylene polyphenyl polyisocyanate.
U.S. Pat. No. 4,490,300 discloses room temperature stable, liquid isocyanates made by reacting diphenylmethane diisocyanate with an aliphatic diol having a pendant aromatic group (e.g., 2-methyl-2-phenyl-1,3-propanediol or phenyl-1,2-ethanediol).
U.S. Pat. No. 4,738,991 discloses organic polyisocyanates characterized by allophanate linkages which are prepared by reacting an organic polyisocyanate, including 2,4'- and 4,4'-methylenediphenyl diisocyanate, with poly- or monohydric alcohol in the presence of an organo metallic catalyst. The catalyst is deactivated upon completion of the reaction using a compound such as an inorganic acid, organic acid, organic chloroformate or an organic acid chloride.
U.S. Pat. No. 4,866,103 discloses a polyisocyanate composition useful for producing elastomers in a RIM process. These polyisocyanate compositions are the reaction product of an alcohol and/or thiol (average functionality of from about 1.5 to about 4 and average equivalent weight of at least 500) with at least 2 equivalents per hydroxyl and/or thiol equivalent of an organic polyisocyanate (including 4,4'- and 2,4'-isomers of diphenylmethane diisocyanate) under conditions such that at least about 20% of the initially formed urethane and/or thiourethane groups are converted to allophanate and/or thioallophanate groups.
Other prior art relating to the preparation of allophanates which contain isocyanates includes British Patent 994,890. This patent discloses reaction of urethane isocyanates with excess diisocyanate either by application of heat alone or by heating in the presence of a catalyst such as a metal carboxylate, a metal chelate or a tertiary amine, until the isocyanate content is reduced to the theoretical level (i.e., that which is obtained theoretically when complete reaction of the urethane groups is achieved).
U.S. Pat. No. 4.160,080 discloses a process for producing allophanates containing aliphatically and/or cycloaliphatically bound isocyanate groups. In the disclosed process, compounds containing urethane groups are reacted with polyisocyanates having aliphatic and/or cycloaliphatic isocyanate groups in the presence of a strong acid. The process is generally conducted at a temperature of from 90.degree. C. to 140.degree. C. for about 4 to 20 hours.
Japanese Patent Application No. 1971-99176 discloses a method for preparing liquid diphenylmethane diisocyanate by reacting diphenylmethane diisocyanate with aliphatic monovalent alcohol.
U.S. Pat. No. 5,319,053 is directed to a stable, liquid MDI prepolymer. This liquid prepolymer comprises an alcohol-based, allophanate modified MDI having an isocyanate content of about 12 to 32.5% and is characterized in that the allophanate is a reaction product of an aliphatic alcohol and a specified isomer composition of MDI containing from 2 to 60% of the 2,4'-isomer, less than 6% of the 2,2'-isomer, and the balance is the 4,4'-isomer.
It is also possible according to another embodiment in U.S. Pat. No. 5,319,053 that this stable, liquid MDI prepolymer has an isocyanate content of 5 to 30%. In this embodiment, the prepolymer comprises the reaction product of the allophanate described above, and an organic material containing at least two active hydrogen groups, and/or a low molecular weight diol. The organic material containing active hydrogen groups may be one wherein these groups are hydroxyl groups, primary amine groups, secondary amine groups, or combinations thereof.
Copending application Ser. No. 08/116,141, filed on Sep. 2, 1993, which is commonly assigned, discloses that RIM processing of the stable, liquid MDI prepolymers described in U.S. Pat. No. 5,319,053 yields molded products which exhibit a high flex modulus. These stable, liquid MDI prepolymers contain allophanate groups and possibly urethane groups.
U.S. Pat. No. 5,319,054 discloses a process for the production of liquid methylene diphenyl diisocyanates in which one equivalent of a diisocyanate is reacted with one equivalent of an aliphatic alcohol to form a diurethane. The diurethane is then reacted with an isomeric mixture of diphenylmethane diisocyanate in an amount sufficient to generate an allophanate having an isocyanate group content of from about 12 to about 30%.